The most common side effects of Paxlovid, a bitter metallic taste and diarrhea, are unpleasant but not dangerous. The truly serious risks come from drug interactions: one of Paxlovid’s two active ingredients powerfully blocks a liver enzyme that processes dozens of other medications, which can cause those medications to build up to toxic or even fatal levels in your body.
The Most Common Side Effects
In clinical trials, the two side effects that showed up more often in Paxlovid patients than in those taking a placebo were an altered sense of taste (5% of patients) and diarrhea (3%). Other reported side effects include headache, nausea, vomiting, abdominal pain, high blood pressure, and a general feeling of being unwell. Most of these fall in the mild-to-moderate range and resolve after the five-day treatment course ends.
The diarrhea rate was only slightly higher than the placebo group’s 2%, which means some of it may be related to being sick with COVID itself rather than the drug. Still, if you’re already dealing with COVID symptoms like fever and fatigue, adding digestive trouble on top can make you feel significantly worse for those five days.
“Paxlovid Mouth”: The Metallic Taste
The bitter, metallic taste that many patients describe is intense enough to have earned its own nickname. It can be constant, lingering between meals, and some people find that food and drinks temporarily mask it while others find everything they eat takes on the same metallic flavor. Staying hydrated is especially important because the taste discourages eating and drinking at a time when your body is already fighting off an infection and potentially losing fluids to diarrhea.
The good news is that it’s temporary. The taste typically fades within a few days of finishing your last dose. In rare cases, reports have documented it lasting up to 34 days, but that’s the exception. For most people it clears up quickly once the drug is out of your system.
Dangerous Drug Interactions
This is where Paxlovid’s risks become genuinely serious. Paxlovid is a combination of two drugs: nirmatrelvir, which attacks the virus, and ritonavir, which exists solely to slow your liver from breaking down nirmatrelvir so it stays active longer. The problem is that ritonavir doesn’t just slow the breakdown of nirmatrelvir. It powerfully blocks a key liver enzyme (called CYP3A4) that your body uses to process a long list of other medications. When that enzyme is blocked, those other drugs accumulate in your bloodstream at concentrations that can be five times higher than normal, or more.
The FDA’s prescribing information is blunt: this mechanism “may lead to greater exposure of certain concomitant medications, resulting in potentially severe, life-threatening, or fatal events.” That language isn’t hypothetical. The interactions are well-documented and predictable based on how each drug is metabolized.
Medications You Cannot Take With Paxlovid
The contraindicated drug list spans several major categories:
- Heart rhythm drugs: amiodarone, dronedarone, flecainide, propafenone, quinidine. Elevated levels of these can cause fatal cardiac arrhythmias.
- Cholesterol-lowering statins: lovastatin and simvastatin. These can be paused for the five-day course, but taking them together risks severe muscle breakdown.
- Sedatives: triazolam and oral midazolam. Excess levels cause extreme, prolonged sedation.
- Certain antipsychotics: lurasidone and pimozide.
- Ergot-based migraine drugs: ergotamine, dihydroergotamine. Toxic levels can restrict blood flow to extremities.
- Gout medication: colchicine, particularly in patients with kidney or liver problems.
- Immunosuppressants: voclosporin. Other immunosuppressants like tacrolimus and cyclosporine require very careful dose adjustments.
- Sildenafil when used for pulmonary arterial hypertension (the dose is much higher than for erectile dysfunction).
The interaction also works in reverse. Certain drugs speed up the same liver enzyme, which can make Paxlovid less effective. The seizure medications carbamazepine and phenytoin, the antibiotic rifampin, and the herbal supplement St. John’s Wort all fall into this category. Taking any of these with Paxlovid can reduce the antiviral to ineffective levels.
Paxlovid also inhibits several other enzyme pathways and drug transporters in the body, which widens the list of potential interactions beyond the most dangerous ones above. If you take any regular medications, your prescriber needs to review every single one before starting treatment.
Allergic Reactions
Severe allergic reactions, including anaphylaxis, are listed as a known risk. Symptoms to watch for include difficulty breathing, swelling of the throat or tongue, hives, and a rapid drop in blood pressure. These reactions are rare but can be life-threatening. Anyone with a known allergy to either nirmatrelvir or ritonavir should not take Paxlovid.
COVID Rebound After Treatment
Some people test negative or feel better while taking Paxlovid, only to have symptoms or a positive test return days after finishing the course. This phenomenon, sometimes called “Paxlovid rebound,” got a lot of attention early on, but the clinical trial data tells a more nuanced story.
A CDC analysis of the original trial data found that viral rebound occurred in about 8% of Paxlovid patients compared to roughly 6% of those who took a placebo. That gap is real but small, and during the later Omicron-era portion of the trials, the difference between the two groups was not statistically significant. In other words, some degree of viral rebound after COVID appears to happen regardless of treatment. Paxlovid may slightly increase the likelihood, but rebound is not unique to the drug.
Rebound episodes are generally mild and self-limiting. They do raise practical questions about how long you remain contagious, which is worth discussing with your doctor if symptoms return after you finish your course.
Kidney and Liver Considerations
Because Paxlovid is processed through both the liver and kidneys, impaired function in either organ changes how the drug behaves in your body. People with moderate kidney impairment typically receive a reduced dose. Those with severe kidney disease or severe liver disease may not be candidates for Paxlovid at all, since the drug can accumulate to unpredictable levels when these organs can’t clear it efficiently. The ritonavir component, which is the source of most drug interactions, is also the component most affected by liver function, making this a particularly important screening step before treatment begins.